Steel used for ships, marine structures, and pressure vessels is formed into a structure having a desired shape by weld bonding. Therefore, it is required that such steel realize high toughness of welded joint portions (e.g., weld metal and heat affected zone) as well as, needless to say, high base-material strength and toughness from the viewpoint of safety of structures.
Hitherto, absorbed energy determined by a Charpy impact test has been mainly used as a standard for evaluating the toughness of steel. In order to enhance reliability, recently, a crack tip opening displacement test (hereinafter, referred to as “CTOD test”) has been commonly employed. A CTOD test evaluates resistance to brittle fracture by performing three-point bending of a test piece having a fatigue crack formed in a toughness evaluation portion and then measuring an opening displacement at the crack tip immediately prior to fracturing.
Since a CTOD test utilizes a fatigue crack, a very minute region serves as a toughness evaluation portion. Thus, if a local brittle zone is present, low toughness may be measured by a CTOD test even though high toughness is measured by a Charpy impact test.
A local brittle zone is likely to be formed in a weld heat affected zone (HAZ) that is subjected to a complex thermal history due to multipass welding of a thick steel plate or the like. A bonded portion (interface between a weld metal and a base material) and a portion in which the bonded portion is reheated to form a dual-phase region (portion in which coarse particles are formed in the first weld cycle and a dual-phase region of ferrite and austenite is formed due to heating by the following weld path, hereinafter, referred to as “dual-phase-region reheated portion”) may become a local brittle zone.
A bonded portion is subjected to a high temperature near its melting point, which increases the size of austenite grains, and is likely to be caused to be transformed into an upper bainite structure having low toughness by the subsequent cooling. Therefore, the matrix itself has low toughness. In addition, brittle structures such as a Widmannstatten structure and a martensite-austenite constituent are likely to be formed in a bonded portion, which causes further degradation of toughness.
In order to enhance the toughness of a bonded portion, for example, a technique in which TiN is finely dispersed in steel and thereby an increase in the size of austenite grains is suppressed or the dispersed TiN is utilized as ferrite transformation cores has been in practical use.
Patent Literatures 1 and 2 disclose a technique of enhancing welded portion toughness by adding a rare-earth metal (REM) to steel in combination with Ti, dispersing fine particles in the steel, and thereby suppressing growth of austenite grains.
In addition, a technique in which an oxide of Ti is dispersed, a technique in which ferrite-core formation capacity of BN is utilized in combination with dispersing of an oxide, and a technique of enhancing toughness by adding Ca and REM and thereby controlling the form of a sulfide are described.
Patent Literature 3 proposes a V-free refined high-tensile steel because, in the case of multipass welding, a brittle zone due to precipitation hardening of V, which is a precipitation-type element, serves as a local brittle zone in a CTOD test and this reduces a critical CTOD value.
However, the above techniques are intended for steel materials having a relatively low strength and less amounts of alloy elements and are not applicable to steel materials having a high-tensile and large amounts of alloy elements because, in this case, a HAZ microstructure does not include ferrite.
Patent Literature 4 discloses a technique for promoting formation of ferrite in a weld heat affected zone mainly by increasing the amount of Mn added to 2% or more. Patent Literature 5 describes a technique for improving CTOD characteristics (CTOD toughness) of a HAZ by making the microstructure of a weld heat affected zone finer by employing a high-Mn type chemical composition, controlling the amount of oxygen to an appropriate value, and thereby increasing the number of intra-granular transformation ferrite cores as well as by controlling a value of a parametric expression consisting of brittle elements such as C, Nb, and V.
However, alloy elements such as Mn are likely to segregate at the center of a slab in a continuous-cast material. This increases the hardness of a center-segregation zone in a weld heat affected zone as well as in a base material and the center-segregation zone becomes a starting point of fracturing. As a result, base-material toughness and HAZ toughness become degraded.
Patent Literature 6 proposes a technique in which a strand having no center segregation is produced by reducing the thickness of the strand by pressing the strand with a plane during solidification subsequent to continuous casting and a microstructure in the vicinity of a weld bonded portion is improved using a complex oxide.
Patent Literature 7 proposes a technique of designing components by determining an average analytical value of the components contained in a microscopic region including segregation of the central portion in a plate-thickness direction located at a position corresponding to the center of a slab and thereby deriving a segregation parametric expression.
In a dual-phase-region reheated portion, carbon concentrates at a region that has reverse-transformed into austenite due to dual-phase-region reheating and thereby a vulnerable bainite structure including a martensite-austenite constituent is formed during cooling, which causes degradation of toughness. Patent Literatures 8 and 9 disclose a technique in which toughness is improved by setting a steel chemical composition to contain low C and low Si and thereby suppressing formation of a martensite-austenite constituent and base metal strength is maintained by adding Cu. In the above technique, strength is increased by precipitation of Cu through an aging treatment, and a large amount of Cu is added. This causes degradation of hot ductility and accordingly deteriorates productivity.
As described above, various factors affect CTOD characteristics. Thus, Patent Literature 10 proposes a steel material with which good CTOD characteristics of a multipass welded zone formed by low-to-medium heat input welding are realized. The steel material is produced by taking comprehensive measures such as control of slab-heating temperature for a continuous casting steel slab such that center segregation is reduced, control of the amount of B mixed into a steel chemical composition, and control of a chemical composition with which formation of a martensite-austenite constituent is suppressed.
Patent Literature 11 describes a technique for improving CTOD characteristics of a multipass welded zone formed with a welding heat input up to 100 kJ/cm at maximum by, in the case of large-heat input welding, making effective crystal grains that are units into which HAZ coarse grains are broken finer and, in the case of low-to-medium heat input welding, setting a chemical composition capable of improving grain boundary hardenability due to a reduction in the amount of a martensite-austenite constituent and addition of a trace amount of Nb, suppressing of precipitation hardening, and reducing the hardness of a HAZ.